CORRELATION OF LOWER PORTLAND FORMATION (LOWER JURASSIC) LACUSTRINE CYCLES IN CORES FROM TWO DRILLING PROJECTS IN SOUTH HARTFORD, CONNECTICUT

Two construction-related coring projects provide a continuous record of the lower Portland Formation in South Hartford. Three primary lithofacies associations (based on seven lithofacies) are used to recognize cyclicity within these strata that is related to climate variation. Well-preserved cores from the current South Hartford Conveyance and Storage Tunnel (SHCST) project are compared to cores from the older (1970’s) Park River Auxiliary Conduit (PRAC) project. The goals of this project are to: (1) describe the facies and cyclicity preserved within the cores, (2) identify which sedimentological and diagenetic features are most useful for correlating between the two generations of coring, and (3) determine the lateral continuity of these features.

The Portland Formation is Early Jurassic in age, and the lower part was deposited within lacustrine environments in the rifted Hartford Basin (central Connecticut). It preserves alternating perennial lake facies (black / gray shale) and playa facies (current-rippled / mudcracked reddish-brown mudstone) with occasional sheet flood deposits (cross-bedded sandstone). These reflect cyclic climate-controlled variations in water inflow into the basin under overall arid conditions. Some perennial lake beds in SHCST cores contain authigenic magnesite crystals that help confirm lake bed correlations among these cores. The playa deposits contain intervals of dolomite (?) nodules or other pedogenic features, as well as sheet flood sandstone beds that can be correlated to neighboring SHCST cores.

Laminated black shale and playa intervals that contain nodules or other evidence of pedogenic alteration are the most consistent and reliable features for correlation among the SHCST and PRAC cores. Sandstone intervals provide robust correlation within each set of cores, but these beds show appreciable variations in bed thickness over the 2.5 km distance between the SHCST and PRAC cores sites. The sandstone is interpreted to have limited aerial extent compared to other facies. Finally, magnesite crystals in laminated black shale can be correlated within the SHCST cores, but magnesite does not occur in the PRAC cores of the same interval. This may not reflect original distribution, but may in fact be related to the decades of outdoor core storage that the PRAC cores experienced.